Boy do I have a stumper...

I've got a '71 F-100 390 that is backfiring through the carb when gas is given to accelerate, especially when going up hills or tight turns, anything that is more effort on the motor. It also happens if you're cruising at any speed on any terrain, and apply more than the lightest pressure to the gas pedal. There's a loud pop that shakes the pedal linkage and gas explodes out of the carb. This problem has started gradually, with no one event that began it, and has gotten extremely bad.

It has a new carb which I am 99% sure is working fine. New points, condenser, power valve. I've seen other people with this same problem on here, and I'm wondering if anybody has any clues. I am at about 7,000 feet so my timing is set at about 10 which per the manual is supposed to be right, it doesn't seem to be missing. The jets are 55s... could these be too big? I tried opening up the choke more, that made it worse. The leaner the mixture, the worse it runs, but it seems likes it's loading up and maybe that is causing the backfires?

When I installed the new carb and set it it ran fine for more than a month or two before this started, and it seems like (this may be all in my head) it started with and might be tied to the cold weather (I'm in Colorado).

This is driving me nuts because it's something really simple and I know I'm not going to find it until I've dismantled the whole truck!!!

Any clues greatly appreciated!!!
Kristen

 

What carb do you have? Sounds like lack of fuel,float level or fuel accel pump. Are they correct? Have you checked vacuum readings lately? Let us know.

 

I have the Autolite 2100, which may not be the right carb for this, although every manual shows it. When I bought it it was a frankentruck with entire systems missing... the engine, as far as I can tell, is probably a '69 but I have no tag. Does anybody know how to date an engine with no tag? It's not the original engine.

Vacuum readings are one thing I have not been able to do because my gage broke the other day. I am going to do that this afternoon. I've seen other people mention the float level, but what I wonder is how could that have changed on it's own? I think the carb is okay, I think it's more likely vacuum or ignition, but it could be anything. This poor truck has been torn apart and put back together by morons several times before this moron had a chance at it! : )

 

Sounds like the accelerater pump diaphragm has torn.

Joe

 

Get engine casting #'s on block, normally on passenger side and post them here. Plugged fuel filter can change level. Let us know.

 

I know the fuel filter is clear... (i have a glass one). After I monkey with it, take the carb off and put it back on, take off some hoses, put them back on, etc., it seems to run fine for a short period of time, even during acceleration. If the accelerator diaphragm was torn, would it be possible for it to backfire only sometimes? It doesn't backfire in that short period of time after I mess with it. It is will run okay until we hit a big hill or something and then the problem comes back. Like maybe something's sticking and by screwing around I'm shaking it loose. I will check the diapragm. Thanks!

 

Only casting I can find is "352"....

 

Where on the passenger side block should I look for this casting number? I know that the casting number is usually the revision or the date of when they started using that part. Is there a casting that shows the year the truck was built at the factory?

 

Is there debri in the fuel bowl? check it out.

 

The carb is clean throughout. I also cleaned and rebuilt the distributor today and played around with different jets, as well as checked the timing, and everything seems to be in order. We've exhausted just about every option! I'm going to have it diagnosed by a friend who is the mystery problem guru, let you know what we find out!

 

hey i have the same problem with my 360 here in iowa, i havent figured it out yet either. it happened with a autolite carb as well as the stock ford one i just bought yesterday. it has to be something else besides the carb.

 

Bad Vacuum advance ?


Timing issues ?

 

That's what I think as well. This carb is just about brand new. Have you checked your vacuum readings? That's one thing I haven't been able to check yet because I don't have a gage. Whatever it is, it's not your distributor or points either probably, I've gone through everything. I'm trying to think big picture now, like maybe a fouled muffler (banana in the tailpipe routine). Timing does not seem to be the culprit, nor are jets or anything else in the fuel delivery system. I am stumped, stumped, stumped.

 

my vacuum advance is brand new, isnt that but the timing issue is a possibility, my timing been messed up for awhile, hope someday i can resolve it.

 

I played with my timing for literally 7 hours today and it made no difference. If I could even get it started, it backfired like crazy. The neighborhood must have thought there was a gunfight going on. I checked my vacuum advance as well and it appears to be functioning normally. I think this is an outside the box porblem, but I see there are lots of other people on here who have a similar issue. Sounds like factory recall to me: why yes, Mr. Ford dealer, OF COURSE I'll take a new motor. No, no trouble at all!